1. Field of the Invention
The present invention relates to a flexible, electrically heatable hose having an elongated electrical conductor, which extends in the helical shape in the hose direction.
2. Discussion of the Prior Art
In a multiplicity of processes for production or conveying of products, flow metering of liquid or gaseous substances, transport of gases or liquids or of products in the form of dust, and for conveying of heat carriers for heat treatment of substances through flexible hoses, it is necessary to supply an amount of heat externally, in order to maintain the temperature of the substances or even to increase the temperature of the substances.
In this case, the maintenance of the temperature of the medium which is carried in a hose is a process in which the proportion of the heat energy which flows to the outside because of the lack or inadequate thermal insulation, and is lost, must be compensated for by a suitable external heat supply.
Thermal insulating materials cannot physically completely prevent the heat loss originating from a heated surface, but can only reduce it. The extent of the reduction depends on the extent of the insulation and on the specific thermal conductivity of the insulating material. However, excessive layer thickness of the insulating material considerably restricts the mobility of a hose system.
In addition to thermal insulation, electrical heating can be provided in the hose in order to maintain a temperature, the effectiveness of which is dependent to a major extent on the efficiency of the electrical heating and therefore on the contact between a heating element and the surface to be heated within the hose arrangement. Electrically heatable hoses are used in many technical fields. In this case, an increase in demand has been observed for particularly flexible hose systems. These systems are used, for example, in the field of automation, for example on robots in the automobile industry.
The fundamental requirements for electrically heatable hoses are a high level of bending strength and torsional strength, with a long life at the same time. A further factor at the same time is the desire for technically proven and economic heating of a hose system.
One heatable hose system is known from GB 2 065 430 A. In this case, an inner hose is sheathed with a layer which produces heat and in which a plurality of electrical conductors are embedded. The heat-producing layer is composed of carbon-doped plastic, as a result of which it conducts electric current when an electrical voltage is applied between two conductors. The resistance of the layer produces heat, which is emitted to the medium being carried in the hose.
EP 0 312 204 A2 likewise discloses the provision of an electrically conductive polymer with a positive temperature coefficient (PTC) as a heat-producing layer in an electrically heatable hose, wherein two electrodes of opposite polarity are arranged in a helical shape and each make electrical contact with the heat-producing layer.
DE 20 2005 004 602 U1 likewise discloses a heated hose line for liquids, which has a central layer composed of an electrically conductive polymer with a positive temperature coefficient. Two heat conductor wires, which extend along the hose, are embedded in the electrically conductive polymer layer, in order to supply voltage to the electrically conductive polymer layer, which is used as a heating element.
However, the hose systems which are known from the prior art have the problem that there is a very major risk of the live electrical conductors losing their electrical contact with the heating element locally when the flexible hose being subjected to very severe and frequent tensile, bending and torsion loads. The contact for heat transmission between the conductor and an inner hose carrying the medium is frequently not ensured, in the event of major deformation of the hose, even in the case of systems in which an electrical conductor is itself used as the heating element.
Furthermore, corrosion can occur on the conductor surface in those conductor sections in which the contact between the electrical conductor and the heating element or the inner hose has been lost by severe and frequent tensile, bending and torsion loads. This corrosion, which results in a greater contact resistance between the live conductor and the heating element, reduces the heat produced in the corresponding area of the inner hose and can thus in some cases result in points with less heating or even no heating. In the event of severe and major deformation, the previously known hoses therefore cannot guarantee uniform heating over the entire hose length.